The present invention relates to a gas dryer which includes a drying drum containing an adsorbent material, and, in particular, to such a dryer in which a primary gas is dried using the adsorbent material and in which a secondary gas is used to regenerate the adsorbent material.
A drum dryer which can be used in a compressed gas system comprises a housing and a quantity of an adsorbent material within the housing by which fluid in the gas to be dried (the primary gas flow) can be adsorbed. The fluid can be aqueous (especially water) or non-aqueous. It will generally be entrained in the primary gas flow in gaseous form (for example as a vapour), although it can be entrained in liquid form (for example as droplets). The adsorbent material can be regenerated for reuse by means of a secondary gas flow which is supplied to the drum to flow over the adsorbent material which is loaded with adsorbed fluid (generally liquid). The secondary gas flow is usually in the opposite direction to the primary gas flow. Such dryers are particularly used in compressed air processes as dehumidifiers to remove vapour from a compressed gas stream which has been cooled after compression stages. The secondary gas flow which is used to regenerate the adsorbent material can be compressed air at elevated temperature as a result of the compression stage, which is fed through the adsorbent material so as to regenerate it.
An example of a drum dryer is disclosed in U.S. Pat. No. 5,385,603, in which an adsorbent material is arranged in on the surface of a cylindrical drum which is arranged to rotate around its longitudinal axis. The adsorbent material can be define a plurality of parallel channels for gas to flow over the surface of the drum, in a direction that is parallel to the drum axis. The drum and the housing between them can provide partitions by which the housing can be divided into different regions for gas to flow through the drum. One of the regions can be for the primary gas flow in which the adsorbent material adsorbs entrained fluid, and the other of the regions can be for the countercurrent secondary gas flow, which is used to regenerate the adsorbent material by desorbing adsorbed fluid. The drum is driven rotationally by a motor positioned at one end of the axis of rotation. The axis is only supported transversely at the end from which it is driven. The dryer has a system of seals located around the periphery of the drum between the drum and the housing, to keep the primary and secondary gas flows separate. Frictional forces between the drum and the housing can place significant load on the motor by which the drum is made to rotate.
The present invention provides a gas dryer in which gas is introduced into the dryer housing into the space between the internal wall of the housing and the external surface of the drum, to flow in that space to the drum gas inlet.
Accordingly, in one aspect, the invention provides a gas dryer comprising a drying drum which is arranged for a gas to flow through it to be dried from a first end of the drum to its opposite second end, a housing in which the drum is located with a space around the drum between the internal wall of the housing and the external surface of the drum, and a housing inlet for the gas to be dried through which gas can be directed into the said space, to flow in the space to the first end of the drum where it can be admitted into the drum for drying.
The dryer of the invention has the advantage that the flow of the gas in the space between the housing and the drum can provide primary separation of liquid that is entrained to flow with the gas prior to admission of the gas into the drum. This enables the separated liquid to be collected before the gas is admitted to the drum and has the advantage that the adsorbent material in the drum is not exposed to the liquid. Separation is enabled as a result of the surface area that is provided on the housing wall for collection (including condensation) of entrained liquid and vapour. This primary separation facilitates regeneration of the adsorbent material in the drum.
Preferably, the inlet for the gas that is to be treated, and the path that the gas flows along between the inlet and being admitted to the drying drum is such that the gas flows in a generally tangential direction relative to the drum axis. This increases the length of the path along which the gas flows. The centripetal forces to which the gas and any entrained liquid are exposed during tangential flow around the dryer drum encourages primary separation of any entrained liquid from the primary gas flow. Preferably, the inlet is configured to direct the gas into the space for substantially tangential flow, rather than being directed straight at the surface of the drum in the housing. For example, the inlet can be formed as a port in the wall of the housing with the axis of the port arranged generally tangentially with respect to the annular space between the housing and the drum.
Preferably, the cross-section of at least one, especially both, of the drum and the housing is substantially circular. This has the advantage of encouraging smooth flow of the gas in the annular space. It is particularly preferred that the cross-section of the annular space be substantially constant around the dryer. Smooth tangential flow of gas in the space between the housing and the dryer can encourage separation of the gas and liquid entrained therewith. A preferred drying drum has a circular cylindrical shape.
An advantage of introducing the gas that is to be treated into the space between the drum and the housing is that it the pressure of the gas can be reduced as it enters the space. This can give rise to a reduction in the temperature of the gas. This can facilitate condensation of any vapour that is present in the gas flow, which can then separate from the gas flow. It can be preferred for the gas to enter the space through an injector so that there is a localised reduction in gas pressure at the housing inlet. The reduced pressure in the space and the resulting reduction in gas flow velocity ensures that reentrainment of deposited liquid is minimised.
Preferably, the external surface of the drum is provided by a metal jacket. This can facilitate loss of heat from the dryer, in turn facilitating condensation of liquid from the gas that is to be treated as it flows in the annular space between the drum and the housing.
Preferably, the ratio of the distance from the inlet to the first end of the drum to the overall length of the drum is at least about 0.25, preferably at least about 0.45.
The dryer will generally include a drain through which collected liquid can drain from the housing. Preferably, the drain is located towards the first end of the drum, and includes a partition at the first end of the drum which defines a primary gas chamber that communicates with the primary region of the drum, and a secondary gas chamber that communicates with the secondary region of the drum, the dryer including a seal by which a pressure differential between the primary and secondary gas chambers can be substantially maintained. A suitable seal might be provided by two sealing surfaces in moving contact with one another, for example in the form of a bearing seal.
In another aspect, the invention provides a dryer in which the drying drum has a primary region for the primary gas to flow through it in which liquid in the gas is adsorbed by the drum as it flows from a first end of the drum to its opposite second end, and a secondary region for a secondary gas to flow through to regenerate the drum by displacing adsorbed liquid, the drum including a drain at a first end of the housing for removing liquid from the housing, and a partition at the first end of the drum which defines a primary gas chamber that communicates with the primary region of the drum, and a secondary gas chamber that communicates with the secondary region of the drum, the dryer including a liquid trap into which liquid in the primary chamber drains, the partition extending into the trap so that, when the dryer is in use, it extends into collected liquid in the trap to provide a seal against gas flow between the primary and secondary gas chambers. The dryer of the first aspect of the invention can include a drain having these features. The dryer may however include drains with other constructions. The drain might for example be of the kind used conventionally to remove collected liquid from a pressurised gas system, such as disclosed in EP-A-8 1826.
Preferably, the dryer includes a first flow guide at a first end of the drum and a second flow guide at a second end of the drum, which define between them a primary region of the drum for a primary gas to flow through the drum, and a secondary region of the drum for a secondary gas to flow through the drum, the flow guides being mounted for rotation about the drum axis so that boundaries between the primary and secondary regions rotate around the drum, the permeable structure of the drying drum and the flow guides allowing the primary and secondary gases to flow through their respective regions of the drum without significant mixing.
Preferably, the first and second flow guides rotate about a common axis. For example, the dryer can include a drive shaft which passes through the centre of the drying drum, with the first flow guide mounted on the drive shaft at a first end of the drying drum and the second flow guide mounted on the drive shaft at a second end of the drying drum.
Preferably, the dryer includes locating means which urges one or both of the first and second flow guides towards the drying drum. This can help to minimise unwanted loss of gas. Preferably, the first and second flow guides are configured such that the leading edges of the flow guides are maintained at a controlled clearance from the respective ends of the drying drum so as to prevent significant egress of gas from the flow guides, while preventing contact of the guiding edges with the respective ends of the drying drum. The clearance will be kept as small as possible, consistent with avoiding contact between the flow guides and the ends of the drum. The actual clearance will depend on factors such as the accuracy with which the drum is manufactured. If the drum can be manufactured with sufficient control, the clearance between the flow guides and the drum might be as small as 4 mm or less, for example not more than about 2 mm, preferably not more than about 1 mm.
The first flow guide can trace a sector having a fixed first angle over a first end surface of the drying drum as the first flow guide rotates. The flow guide can include a primary gas inlet for the gas that is to be dried; a coalescing device can be included in the primary gas inlet to collect any liquid that is entrained in the flow of the primary gas, for example in the form of a foam material as known for this purpose.
The first flow guide can be divided into primary and secondary gas chambers for (a) the gas that is to be dried as it passes through the primary region of the dryer drum, and (b) the gas that passes through the secondary region of the drum to regenerate it. Preferably, the second flow guide traces a sector over its end surface of the drying drum as the guide rotates, the sector corresponding approximately to the sector traced by the secondary gas chamber of the first flow guide as the first flow guide rotates.
Preferably, the dryer includes a support extending between an internal surface of the housing and a side wall surface of the drying drum, to fix the drum against movement relative to the housing and to support the drum in the housing. The support will generally contact the side wall surface of the drum at a point between its ends. Preferably, the support bears substantially the entire weight of the drying drum. This has the advantage that it enables problems associated with bearing the load of the drum at its ends on a rotating shaft, as in the dryer disclosed in U.S. Pat. No. 5,385,603, to be avoided. The provision of movable flow guides in the dryer of the invention allows the drum to be fixed. This can simplify control over the clearances between the drum and the flow guides, since the location of the more massive drum component can be controlled by virtue of it being fixed to the housing. Having the housing bear the entire weight of the drum facilitates control over the clearances between the drum and the flow guides.
To facilitate manufacture, the housing can have a first part and a second part joined at an interface, with the support is attached to the housing at the interface. Preferably, the housing parts have substantially the same shape and configuration. This reduces inventory during manufacture. The support can be secured to the housing at the interface between the two parts thereof. An example of a suitable support comprises a substantially right angular flange. When the inlet for the gas to be dried is formed as a port in the wall of the first housing part, the corresponding port in the second housing part (of the same configuration as the first housing part) can be used as an outlet for gas that has been dried and has passed through the drum. When the port on the second housing part is not required for outflow of gases, a blanking piece can be provided within it to close it against gas flow.
Preferably, the support extends continuously around the drum and the internal surface of the housing. Such a support can divide the housing into first chamber and second chambers, such that gas can only pass between the first and second chambers through the drum. The inlet for the gas to be dried will generally be provided in one of the housing parts and the outlet for the gas after it has been dried will generally be provided in the other of the housing parts. Generally, at least one, and preferably both, of the inlet and the outlet will be provided in the side walls of the housing parts.